Friday, July 22, 2016
Then it gets very technical: Sections 4 and 5 amounted to finding orbits from a few points, which amounts to interpolating conics. Section 6 is a bit more interesting, finding position as a function of time (as opposed to just orbital shape) via clever use of Kepler's law of equal areas in equal times. Section 7 involves falling bodies in central potentials. Section 9 involves precessing orbits. Section 10 involves objects on curved surfaces, and also pendula. Alas, all of the methods here are impenetrable.
Section 11 is interesting. He discusses how to convert 2-body problems into 1-body problems, justifying the application of his previous results (on 1-body problems) to real systems. Then he takes on the 3-body problem. Given the difficulty of the problem he isn't able to do much with the gravitational problem, but he does prove some interesting results for the n-body system of particles interacting via 2-body forces proportional to the products of particle masses and the separation vector between the particles. Basically it's a system of masses and springs and it's completely solvable. So, like any physicist, Newton derived results for masses and springs because that's what he could do. Don't feel bad, Isaac, we have mostly done the same in the centuries since.
When he gets to the 3-body gravitational potential, he argues ("proves" is too strong of a term) that if one mass is far larger than the rest you can ignore the interactions of the other two bodies with each other, or the way that they perturb the motion of the larger body. After that, he argues that his methods can be applied to a 3-body system that bears a remarkable resemblance to the sun, earth, and moon. However, he kept it quite abstract, deferring an open mapping to the sun/earth/moon system until Book 3. According to the commentary by the translator, the reason was that Newton's contemporaries were critical of the idea of mysterious attraction at a distance.
In section 12 he proves that for gravitational interactions spheres can be treated as point objects. In section 13 he considers a point interacting with a surface if the particle-particle interactions are something other than 1/r^2. He derives a lot of results that would not be out of place in a modern text on intermolecular forces.
Section 14, the last section of Book 1, involves small particles moving in stratified media with piecewise-linear potentials. What he is doing bears a remarkable resemblance to ion optics (which I spent some time working with in college) but he wants to derive the optics of light. It's a precursor to his book Opticks.
I'm not going to read books II and III. Book II is chock full of wrong results and Book III just doesn't interest me because the essential result is already present.
Still, although I wound up skimming a third of it rather than reading the whole thing carefully I am glad that I read it. I hope to teach my department's class on the history of physics in a few years, and having at least surveyed Newton is useful. More importantly, I want to focus on the topic of relative motion, and Newton's most important arguments there concern the spinning bucket, which is in an early section that I actually read. The bucket argument influenced Mach and Einstein, so it's essential to the course that I would like to teach.
Thursday, July 21, 2016
At some point I gave up on trying to get the details of each step of each proof and instead just figure out the chain of ideas from one proposition to the next. Section 2 went from fundamental results that I can articulate to technical results whose motivation was hard to follow at the time. In fact, even when I think I have unpacked them I then find myself wondering if I am missing the point. This is not a book that I will be able to digest, only tour. But in section 3 he uses these results to conclude that conic orbits with the sun at the center imply that objects are attracted to the sun via an inverse square power of the distance from the sun, and that the inverse square law implies Kepler's laws. That alone would guarantee his immortality, but he still has more to do. Let's see what he has for us next.
Wednesday, July 20, 2016
The first 7 corollaries are great. The guy derives the inverse square law of gravity for circular orbits, assuming Kepler's law (T^2 ~ R^3). Awesome.
Then Corollary 9 threw me through a loop. The translators used the term "mean proportional" rather than "geometric mean." But I sorted it out.
This book is a hard slog, but worth it.
Tuesday, July 19, 2016
In the second proposition he just shows that if you observe something moving in such a way as to sweep out equal areas in equal times (looking at arcs drawn about some designated center) then it must be acting under a centripetal force.
In the third proposition he argues that if a body is tracing out equal areas in equal times with respect to some accelerating object, then the first body must be under the combined influence of a centripetal force from the second object and also the same accelerative force as the second object is feeling. In other words, Newton translates his results to non-inertial frames.
So far the man has built laid down basic laws of motion, described a painstaking experiment, built up calculus, derived one of Kepler's Laws, and developed some physics for non-inertial frames. This is why physicists regard him as something of a deity. He's blending together a bunch of techniques and fundamental concepts, any one of which could easily be a claim to some fame in the history of physics, and at the same time he's blending together skill sets that few physicists will ever master simultaneously.
This is why we love him.
Monday, July 18, 2016
Corollaries 5 and 6 show that the relative motion of a system of interacting bodies is unaffected if we move to a different inertial reference frame, or to a non-inertial reference frame at constant acceleration.
The Scholium shows us what a genius he is. He cites prior work by Huygens, Wren, and Wallis on collisions, but notes that they only worked out elastic collisions. He works out an example with an inelastic collision of two hard objects on pendula, shows how to estimate the effect of dissipative forces in a self-consistent manner, and then describes an experiment that he did to test his calculations (involving a 10 foot pendulum and errors no larger than 3 inches for the maximum height reached by objects, i.e. 2.5% error).
Having showed his skill as an experimental, he uses symmetry arguments and the impossibility of perpetual motion to demonstrate the validity of the Third Law for attractive forces, including a thought experiment on the stability of the earth as a self-gravitating object. Since the Third Law can be shown (in the Lagrangian formalism) to arise from translational invariance, I like his use of symmetry arguments here. It may be the earliest precursor of Noether's Theorem.
Then he talks about the difference between absolute motion and relative motion, absolute motion being motion relative to space and relative motion being relative to another object. He makes the important point that you cannot discern absolute motion by looking at the relative motion of two different objects because you don't know the absolute motion of either. However, he then explores his famous bucket thought experiment to argue that there are some cases in which absolute acceleration can be inferred. I've already discussed that thought experiment and Mach's reaction to it, so I have nothing to add here. I will just say that the real significance of the Principia is not his conceptual understanding of physics (a modern pedagogue could easily find much to scold him for) but rather his ability to add new ideas to a big, bold, framework and then apply those ideas unflinchingly and with no reluctance to generalize, and extract predictions for the motion of the planets.
Next up: His axioms, or laws of motion.
I learned of this article from a piece by Joshua Kim at Inside Higher Ed. Kim argues that most people in instructional design hold opinions on lecturing that are more nuanced than those that the Atlantic article is responding to. That may be true, but the currents that Gross-Loh highlights are nonetheless real. I've submitted a comment saying that; I won't regurgitate it here because it's nothing I haven't said a million times before and it will show up soon enough at IHE.